Compression rings form a seal between a piston and a cylinder wall. They are adapted to use combustion pressure to force the ring against the cylinder wall and against the bottom edge of a ring groove. Typically, a top ring is the primary seal with a second ring being used to seal any small amount of pressure that may reach it.
During a power stroke, the pressure generated by the ignited and expanding air/fuel mixture is applied between the inside of the ring and the piston groove. This forces the ring into full contact with the cylinder walls. The same combustion pressure is applied to the top of the ring, forcing it against the bottom of the ring groove. The combustion pressure and the compression ring act together to form a ring seal.
Oil is constantly being applied to the cylinder walls. The oil is used for lubrication as well as to clean the cylinder wall of carbon and dirt particles. This oil bath also aids in cooling the piston. Controlling this oil bath is the function of the oil ring. The two most common types of oil rings are a segmented oil ring and a cast-iron oil ring. Both types of rings are slotted so that excess oil from the cylinder wall can pass through the ring. The oil ring groove of the piston is also slotted. After the oil passes through the ring, it can then pass through slots in the piston and return to the oil sump through an open section of the piston.
Those skilled in the art will appreciate that the pistons of internal combustion engines in today's modern vehicles are generally provided with three sets of piston rings for preventing, between the pistons and cylinder bores, leakage of gas to the crankcase, and of oil to the piston head.
As a practical matter, it is well known that the piston rings of modern engines, although substantially improved over engines of prior vintage, are in some ways still lacking. For example, the upper compression ring is designed one hundred percent for the sealing of the gases of combustion to prevent their entry into the engine crankcase. Generally the lower compression ring is designed to provide about forty percent of the noted gas sealing function, and approximately sixty percent of an oil scrapping function. The latter prevents oil from traveling up to the top of the piston head to create the classic smoking tailpipe or “blue smoke” syndrome. Finally, most modern pistons include a bottom oil control ring that includes at least one rail used for aggressive scraping of oil to force the same back into the crankcase. Normally sharing the bottom piston ring groove with the at least one rail is an expander ring formed of an undulating, sinusoidal-shaped spring steel for the purpose of loading the rail appropriately, so that the rail may be effective in its scraping function as the piston reciprocates within its cylinder bore. Hence the combination of the rail and the expander is referred to as an oil control ring.
It will thus be appreciated that various piston rings have unique design functions for addressing either of the noted prevention of leakage of gas to the crankcase, or of oil to the piston head. Generally, as the rings wear during their continuous scraping against the cylinder walls and associated rocking within piston ring grooves, issues of blow-by of gases into the crankcase, and oil leakage into combustion chamber areas, become significant. Most rings incorporate a tangential tension in their initial structure that can generate a force (as measured by a spring band) against the cylinder walls. Unfortunately, this force does not vary, and tends to apply the same force on both upward and downward strokes of the piston.
Particularly with respect to the scraper function of the bottom oil control ring, it would be quite desirable to provide a variable oil ring compression control against the cylinder walls including a variable tension oil ring assembly robust enough to power cylinder G-forces that would add strength and durability to all pieces.